So what is your idea ? Should we just give up doing aikido because you assume that there is nothing to learn anymore ?

Sorry to disappoint you, but I don't value your opinion too much.

To me you are commiting a few important mistakes.
1- You believe that there is not much to find beyond what you saw.

2- The differences that you perceive in the learning methods do not always mean that the essential content is different.

3- You are right to insist that feedback is necessary in the learning process, and sparring is a possible method that can provide it (as Tomiki sensei structured it for example). However, other methods are also valid and efficient. For example, Hikitsuchi sensei always emphasised that practice should be made in a spirit of shinken shobu, which means that the practitionners must put themselves in the proper state of mind to push themselves to improve and eliminate mistakes mercilessly.